Usually, composite material fan blades, in particular fan blades made of carbon fibers, are made from a stack of preimpregnated unidirectional plies that are placed in a mold with successive plies having different orientations, and with the stack then being compacted and polymerized in an autoclave. That technique is very difficult and requires the ply-stacking operations to be performed manually, which is lengthy and expensive.
Proposals have also been made to prepare preforms woven using dry fibers which are subsequently assembled together by stitching, prior to being impregnated with resin that is injected into a closed mold. An alternative consists in making a single woven preform which is mounted with one or more solid inserts prior to injection. Nevertheless, those solutions (U.S. Pat. Nos. 5,672,417 and 5,013,216) present the drawback of requiring a plurality of parts to be assembled together and of creating assembly zones which are likely to become zones of weakness, e.g. due to delamination, and that is very harmful in terms of mechanical strength, and particularly ability to withstand impacts.
To overcome those drawbacks, French patent document FR 2 861 143 proposes making a preform of three-dimensionally woven fibers or yarns that suffices on its own after injection (optionally after being cut out), to form the final part constituting all of the portions of the turbomachine blade, without having recourse to using inserts or any other fitted elements.
Nevertheless, under those circumstances, regardless of the origin of the polymerized preform (a preimpregnated laminate or a three-dimensionally woven preform), after the intermediate part obtained at the end of injection has been unmolded, there still remain various operations that need to be performed in order to obtain the final part.
These various operations include accurate machining, in particular of the outlines of the leading edge, of the trailing edge, and of the root. These zones need to satisfy very precise structural dimensions. Thus, particular attention is required for the bearing surfaces of the blade root, i.e. surfaces that are subjected to high levels of stress during rotation because they come into contact with the flank of the cavity in the disk that receives the root. In particular, contact wear, or “fretting”, occurs between these contacting surfaces as a result of repeated rubbing of one part against another, the resulting friction forces damaging the material by generating heat and leading to various fatigue processes.
Also, amongst the subsequent operations, various protector elements are put into place to reinforce the thermomechanical strength of the composite blade. Thus, metal protection is secured to the leading edge, e.g. in the form of a titanium part that is bonded over the entire surface of the leading edge and over a front portion of the outside surfaces of the pressure-side and suction-side walls. The outside face of the pressure-side wall is also reinforced by putting into place a protective film that can be made of synthetic material (e.g. polyurethane) and that is bonded on the intermediate part directly by adhesive.
When all of those operations need to be performed on each blade of a fan, and possibly on all of the blades in a plurality of low-pressure compressor stages, that leads to manufacturing times that are relatively lengthy and that are significant, economically speaking.
Furthermore, when machining the polymerized preform, the zones in question can be weakened because the machining cuts through some of the initial woven yarns of the preform, and in particular the warp yarns.